The discovery of ordered mesoporous silicate materials by the Mobil researchers in 1992 [1] with high surface area (700 - 1500 m²∙g⁻¹), tuneable pore size (2 - 20 nm), and large pore volume (0.4 - 1.0 cm³∙g⁻¹) has attracted great interest for their potential applications in the fields of catalysis, chromatography, sensing, shape selective catalysis and support material [2] [3] . Since their discovery, two of the stable members of this family, MCM-41 having unidimensional pore structure and MCM-48 with three-dimensional pore system have been exploited extensively in the field of catalysis. The scope of application of mesoporous materials is further broadened with the advent of SBA-n molecular sieve and SBA-15 in particular is being widely studied [4] . However, studies on the use of metal free siliceous materials in catalysis are rare and limited. Particularly, purely siliceous framework possesses substantial amounts (~30% - 40%) of silanol (defect sites) groups [5] . And various active functionalities have often been implanted via these hydroxyl groups to enable these silicates to be catalytically active. Notably few interesting reports are available on the oxidation behaviour of silica based materials. Morasas and Harrington have reported the oxidative and hydroxylative conversions of polar aromatic compounds over quartz surface [6] . And the peroxidative activity of silica is chiefly attributed to the polar nature of the silica surface [7] . Similarly, the photocatalytic decarboxylation of organic compounds catalyzed in the presence of mesoporous silicas has studied by Itoh et al. [8] . Recently, we have reported the oxidative ability of mesoporous silica materials towards lignin model compound under microwave irradiation [9] [10] . It has been demonstrated that α- methylvanillyl alcohol (apocynol) can selectively be transformed to either acetovanillone or 2-methoxybenzo- quinone, using mesoporous silica and H₂O₂ as oxidant under microwave activation. These motivating results further stimulated us to continue the study on the reaction under conventional heating rather than microwave activation, as the former was less expensive and industrially important.

Among the lignin model phenolic monomer, Apocynol, 1-(4-hydroxy-3-methoxyphenoxy)-ethanol is commonly studied over different catalysts in order to understand the reaction pathway, which can possibly be extended to lignin based biomass. Recently, much attention is focussed on selective transformation of lignin derived fragments to fine chemicals through catalysis [11] . Wozniak et al. [12] have studied the liquid phase oxidation of apocynol over potassium nitrosodisulfonate (Fremy’s salt) and corresponding benzoquinones were selectively (~80%) obtained over a reaction period of 2 h. Similarly, 2-methoxybenzoquinone was obtained as the common product with maximum selectivity of 50% by using Co(salen) catalysts [13] [14] . Realising the importance of benzoquinones as precursors in various organic reactions, currently the possible generation of quinones from lignin based guaiacyl units is being investigated [15] . In the present study, we report on the unusual oxidation behaviour of the siliceous mesoporous materials towards apocynol using H₂O₂ as an oxidant under mild reaction conditions with highest selectivity towards 2-methoxybenzoquinone. For a comparison commercial grade mesoporous silica (Silica-5, Grace, Germany) was also used as catalyst as received.

The XRD patterns of the calcined mesoporous silica catalysts are shown in Figure 1. XRD patterns of calcined, MCM-41 (Figure 1(a)) and MCM-48 (Figure 1(b)) show a strong reflection observed in between 2θ = 2.2˚ - 3˚ in addition to several weak reflections in the range of 2θ = 3˚ - 7˚ indicated the formation of mesoporous structure. Similarly, in case of SBA-15 (Figure 1(c)), a strong reflection at 2θ = 1.06˚ and several weak reflections between 2θ = 1.5˚ - 3.0˚ confirms its structure. The BET surface area and pore size were determined to be 978 m²∙g⁻¹ and 2.6 nm for MCM-48 while that for MCM-41 and SBA-15 was found to be 850 m²∙g⁻¹, 2.5 nm and 940 m²∙g⁻¹, 6.1 nm respectively [17] .

Figure 2 shows the FT-IR spectra of calcined MCM-41 (a) MCM-48 (b) and SBA-15 (c) respectively. All the calcined spectra, show absence of the characteristic band for surfactant template molecule at 2928, 2854 and 1490 cm⁻¹ leaving behind other characteristic peaks of mesoporous material. In addition to above features, absorption bands noticed at around 1072, 808 and 460 cm⁻¹ are originating from the asymmetric, symmetric stretching and bending vibration for Si-O-Si bonds [18] respectively. A strong band at 966 cm⁻¹ seen clearly, in all the spectra, is attributed to terminal Si-O stretching of silanol groups. The occurrence of broad bands between 3000 - 3800 cm⁻¹ in case of all the samples indicates the presence of surface hydroxyl groups, characteristic of mesoporous structure [19] . It is worth mentioning that both MCM-41 and MCM-48 show intense band in this range compared to SBA-15. This is attributed to the abundantly presence of incompletely condensed silica as evidenced from ²⁹Si MAS-NMR data [20] . The predominant presences of silanol (≡SiOH) groups impart hydrophilic nature to these MCM-n type materials.

We have undertaken a detail investigation on the oxidation of apocynol, 1 (Scheme 1), catalyzed by different catalysts under varying reaction conditions such as temperature, substrate: oxidant, duration and the results are summarized in Table 1.

Over all the silica catalysts used, it was noticed that within a very short period (15 min) major substrate was reacted yielding both 2 and 3. Acetovanillone is the early oxidation product [21] , and 2-methoxybenzoquinone is believed to be formed by the generation of phenoxy radical and subsequent cleavage of side chain [22] .

Scheme 1. Reaction profile of oxidation of apocynol (1).

Reaction conditions: 1.0 mmol apocynol, 3 mmol H₂O_2, 5 mL acetonitrile, 100 mg catalyst at 313 K, Duration 15 - 180 min.

Interestingly, about 98% of substrate conversion was achieved over MCM-41 with best quinone selectivity (~84%) among the catalysts screened. In general an optimum conversion and selectivity was obtained within 30 min of reaction. Continuing the reactions beyond 30 min resulted in an increase of substrate conversion with fall in selectivity, when MCM-48 and SBA-15 were used as catalysts. However, MCM-41 maintained a steady increase in conversion and selectivity.

It is well evident that apocynol is quite stable irrespective of 3 h of heating in the solvent (entry-6). And in presence of H₂O₂ (entry-5), 1 showed high conversion resulting in 2-methoxybenzoquinone (2), acetovanillone (3) along with high molecular weight (unidentified) products. Additional products detected in the higher retention time, are attributed to the possible polymerisation of quinone to form high molecular weight compounds. In order to understand the influence of nature of silica and porous structure, Silica-5 having high surface area with irregular porous structure was employed as catalyst, which gave a lower substrate conversion with nearly equal selectivity towards 2-methoxybenzoquinone and acetovanillone (entry-4). These results indicated the fact that an ordered porous silicate surface as well as the pore architecture governs both the conversion and selectivity.

Another significant feature of this reaction has been, with increase in reaction duration the apocynol conversion remained almost same, while significant fall in acetovanillone yield. In order to understand this aspect, we further investigated the reaction profile by varying substrate: oxidant amount. Representative results of MCM-48 catalyst are presented in Table 2. It is well evident that with increase in the oxidant amount the selectivity of the 2-methoxybenzoquinone was enhanced with drop in acetovanillone yield. These observations point toward that 2-methoxybenzoquinone is the stable product which is formed at the expense of acetovanillone and similar trend was observed over other catalysts studied. Based on the results obtained, the product formation is likely to occur through a plausible pathway (Scheme 2). The polar surface along with high surface area favours the adsorption of both apocynol and hydrogen peroxide (H₂O₂). Subsequently, binding of H₂O₂ onto the -OH group leads to activation of the oxidant resulting in possible formation of hydroxyl radicals. The hydroxyl radical formed either abstracts a phenolic hydrogen atom from the phenolic substrate, 1 (path-1) and produces a reactive phenoxy radical, 4 or it abstracts a proton from the benzylic carbon (path-2), forming the reactive radical, 6. By the path-1, 4, transformed to intermediate, 5, through addition of another hydroxyl radical. Intermediate 5, undergoes rearrangement to form 2-methoxybenzoquinone, 2. On the other hand, through path-2, intermediate 6 undergoes deprotonation via another hydroxyl radical leading to the formation of acetovanillone, 3.

The unlikely oxidation behaviour of the mesoporous silicas is attributed to the abundance of surface hydroxyl groups, polar internal surface, high surface area and pore geometry. At this juncture the facts suggest that,

Scheme 2. Probable oxidation pathway of (1) over silica catalysts.

Reaction conditions: 1.0 mmol apocynol, 5 mL acetonitrile, 100 mg catalyst at 313 K, duration 180 min.

apocynol (1) is susceptible to oxidation in presence of H₂O₂ alone; and the reaction proceeds in an uncontrolled way resulting in several high molecular weight products (not identified). On the other hand, mesoporous silicates along with H₂O₂ catalytic system produced 2-methoxybenzoquinone predominantly, and the reaction was much controlled over silicate catalysts and hydrogen peroxide (H₂O₂) oxidant system.

In conclusion, we have demonstrated a clean, efficient catalyst system for the selective oxidation of an important lignin model phenolic monomer. Metal free mesoporous silicas along with environmentally benign oxidant, H₂O₂, were found to be highly efficient catalytic system for the chosen oxidation reaction, and quinone was selectively obtained under mild reaction conditions. These results open up for further study to evaluate and understand the oxidative nature of mesoporous amorphous silica materials.

We wish to thank Department of Science and Technology, Govt. of India for the financial support under FAST track (SR/FT/CS-93/2007) and PURSE scheme. Authors appreciate many meaningful discussions with Dr. A. Sakthivel, Department of Chemistry, Delhi University, relating the reaction.

RojalinRojalin Sadual,Sushanta K.Badamali,Sudhir E.Dapurkar,Rajesh K.Singh, (2015) Unusual Oxidation Behaviour of Mesoporous Silicates towards Lignin Model Phenolic Monomer. World Journal of Nano Science and Engineering,05,88-95. doi: 10.4236/wjnse.2015.53011